1. Field of the Invention
This invention relates to transducers for sensing power line voltage and current components of alternating current electric energy, and more particularly to such transducers for sensing the line currents and producing corresponding analog signals for use by measuring circuits indicating current or electrical energy parameters of AC electric energy usage.
2. Description of the Prior Art
Current measurements are required for various reasons in AC electric power distribution. Both current responsive and voltage responsive transducers are required in the extensively required power line measurements of electric energy required for billing purposes. Induction watthour metering devices for alternating current electric energy measurement are almost exclusively used by producers of electric energy for measuring consumption by separate energy users. Typically, watthour meters are used for indicating consumption in kilowatt-hours. The watthour meters are usually of the electromagnetic induction type having voltage and current coils and a rotating disc driven by fluxes from the two coils. These induction type watthour meters are recognized as having high degrees of reliability and accuracy, being available at reasonable cost, and being capable of operation in widely varying extremes of temperature and other environmental conditions while providing highly accurate and reliable operation.
It is also known to measure alternating current electric energy quantities such as kilowatt-hours, volt-ampere hours, reactive volt-ampere hours, with electronic measuring circuits. These measuring circuits require the voltage and current components of an electric energy quantity to be measured to be sensed by electrical transducers. Voltage and current responsive analog signals are produced at the outputs of the transducers at low level signal magnitudes. Preferably, the analog signals are isolated from the higher magnitudes of the sensed power line voltages and currents, and the analog signals are required to be compatible with the low level signal limitations of electronic measuring circuits. it is known to utilize potential and current instrument transformers to provide analog signals proportional to the voltage and current components of the electric energy to be measured.
Various types of known electronic energy measuring circuits are adapted to receive the voltage and current analog signals produced by the voltage and current responsive transducers. Examples of such electronic energy measuring circuits and devices are described in U.S. Pat. Nos. 3,343,084; 3,764,908; 3,864,631; and 4,077,061, all assigned to the assignee of this invention. Typically, the voltage and current responsive inputs to the AC electric energy measuring circuits are usually provided by instrument transformers producing analog signals proportional to the line voltage and line current components of the electric energy quantity to be measured. The electronic measuring circuits are operable at the small or low level signal magnitudes while the electric power line voltage and currents are several magnitudes larger. Therefore, it is generally required of the sensing transducers providing voltage and current responsive analog signals to the measuring circuits, that they have large transformation ratios. In the case of current sensing transducers the outputs must be linear over wide ranges of line current input values.
In typical electric energy measurements at an electric power utility customer location, sixty Hertz AC electric power is delivered at substantially constant line voltages of either one-hundred and twenty or two-hundred and forty volts defining the line voltage components of the electric energy quantity to be measured. On the other hand, line current, which defines the current component of the electric energy to be measured, varies considerably, typically in a desired linear measurement range from one-half ampere to two hundred amperes or in a current variation ratio of approximately one to four hundred. Accordingly, standard potential instrument transformer arangements often can provide ractical voltage sensing transducers. Current instrument transformers receiving the aforementioned wide current input variations and producing the low level signal outputs often require structural arrangements which are of substantial size and cost. When it is desired to manufacture electronic AC energy measuring circuits and devices which are relatively compact and comparable in cost to the aforementioned conventional induction type watthour meter, the voltage and current sensing transducers often present substantial contributions to the overall size and cost of such devices. As is known in accurate current instrument transformer transducers, the ampere turns of the primary and of the secondary must be equal, and since maximum current levels and the primary reach two hundred amperes, the primary and secondary winding sizes become substantial in order to produce linear low level signal outputs. Thus, the current transformer transducers become quite bulky and are relatively costly.
Another consideration in using inductive or transformer types of current transducers is the non-linearity of the magnetic materials used for magnetic cores in such transducers. Less expensive magnetic core materials tend to have decreased linearity characteristics so that the output signals therefrom are not consistently proportional over the wide ranges of sensed line currents. The linear response of current sensing transducers having a magnetic core and a primary winding carrying the line current to be sensed can be improved by the use a first or sensing secondary winding and a second or compensating winding which is used to oppose the flux in the core produced by the primary in resonse to the output of the sensing secondary winding. By maintaining the magnetic flux of the magnetic core near zero and compensating for virtually all of magnetic flux produced by the primary current, a more linear response is provided. One problem in this technique occurs when measuring large power line currents over wide dynamic ranges because the ampere turns of the primary winding must be matched by the ampere turns capability of the compensating secondary winding. If the compensating secondary winding is to develop current responsive analog signals for application to the inputs of electronic measuring circuits, with a typical low level signal current value in the order of five milliamperes, a single turn primary winding carrying a current value in the order of two hundred amperes would effect a magnetomotive force of two-hundred ampere-turns so that a compensating secondary winding would require approximately forty thousand winding turns to produce the required bucking ampere turns. Such a current sensing transducer would be of substantial size and cost.
in U.S. Pat. No. 2,428,613, a single turn primary current transformer is disclosed having a ring-shaped core, a first or sensing secondary winding having outputs applied to an amplifier with the amplifier having an output applying a controlled current to a compensating secondary winding so that the ampere turns produced by the compensating secondary winding balances the ampere turns produced by the single turn primary winding. While this device accomplishes the improvement of linear respose, the physical size and cost thereof are substantial for the reasons noted herinabove when the primary current reaches large power line current magnitudes and the output current resonsive analog signals are required to be of the low level signal character. In U.S. Pat. No. 3,539,908, an instrument type transformer arrangement is described for transforming both polarities of a unidirectional current rather than for producing low level current responsive analog signals in response to an AC power line current. A saturating core is described wherein a plurality of secondary windings are provided thereon with the ampere-turns of selected ones of the secondary windings being produced to oppose the ampere turns produced in the saturating core by a single turn primary winding. In British Specification No. 227,910 filed Oct. 23, 1923, a protective relaying device is disclosed having two magnetic cores sensing the current through the centers thereof provided by a power line current flow. Two current transformers are formed in which the current flowing from one through another will produce and the core of the another transformer a magnetic flux equal and opposite to that produced therein by the primary single turn winding thereof when the conditions of the power line circuit are normal. A protective relay circuit is energized when the flux in the other current transformer core becomes unbalanced due to a fault condition of the power line. This patent does not disclose a transformer having a compensating secondary winding for producing a current responsive analog signal proportional to a line current component.
Accordingly, it is desirable to provide a current sensing transducer which is highly reliable and accurate and having relativley simple mechanical and electrical configurations utilizing only a single magnetic core and a simple amplifier arrangement for generating a compensating current which also provides an output signal proportional to the line current being sensed. It is desired that the output signal from a current responsive analog signal which is proportional to the sensed line current over a range of current variation in the order of one to four hundred while having a linearly proportional response thereto by low level output signal values suitable as inputs to electronic current or energy measuring circuits.